The present invention relates to the irradiation arts. It finds particular application in the field of product sterilization, disinfection, and radiation treatment and will be described with particular reference thereto. However, the present invention is applicable to a wide variety of other applications including, but not limited to, food and spice treatment, plastics modification, x-ray imaging, genetic modification, and other fields in which controlled doses of radiation are advantageous.
Products are typically irradiated by being conveyed past a radiation source, such as cobalt rods, electron beam accelerators, or x-ray sources. Cobalt rods are effective, but cannot be turned off for maintenance in the treatment vault. Rather, they are mechanically immersed in heavy water. Spent cobalt rods are changed and stored deep in the heavy water. Accelerated electron beams are easy to control, but have limited penetration power relative to x-ray or xcex3-ray radiation.
X-rays are high energy photons that are produced as a result of accelerated electrons interacting with a target. Typically, metals such as tungsten or tantalum are used. To produce x-rays, free electrons are generated, such as by being boiled off of a filament. The electrons are accelerated in a vacuum through a potential to a desired kinetic energy toward the metal target. The accelerated electrons interact with the electrons naturally present in the target metal. As the electrons interact, some of the kinetic energy of the incoming electrons is transferred into the electrons of the target metal perturbing them into higher energy states. Over time these electrons decay back to their lower energy states releasing energy in the form of x-rays.
X-rays have been found to be very useful in the sterilization of products. This type of high energy radiation, in sufficient doses, kills most all types of living organisms. This includes parasitic bacteria and viruses which have the potential of making people ill. This is useful for sterilizing food meant for consumption, as well as other products such as medical instruments. Of course there is no chance of residual radiation with x-rays, so the product is safe afterwards, and will not harm the consumer as a result of being irradiated.
One of the biggest problems with x-ray production is that not all of the energy of the incoming electrons is converted into x-rays in this manner. The majority of the energy is lost to non-useful collisions and converted into heat. Typically, the best systems convert approximately 15% of the kinetic energy of the incoming electrons into x-rays, i.e. approximately 85% of the energy is converted into heat. This amount of heat is sufficient to destroy or damage the target. In order to conserve the integrity of the target, and thus, the system, sufficient heat is removed to maintain the target below a preselected maximum temperature.
Different types of cooling systems are employed. Relative movement between the electron beam and the target permits heated spots of the target to cool between electron beam irradiations. In high energy applications, the electron beam returns before cooling is complete and heat builds to target damaging levels. Some x-ray systems have a fluid coolant that flows over the target, transferring the produced heat away from the target. Problems with this type of system are low efficiency of the cooling system and short life of the target. Typically, the fluid used is water which flows over the metal target. Over time and extreme stress, the target corrodes.
The present invention presents a new method and apparatus that overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, a product irradiation device is given. Products to be irradiated are propagated upon a conveyer which passes through a region that is irradiated by x-rays converted by a target from high energy electrons accelerated from an accelerator. A radiation shield protects the area and a control room from ambient radiation. The target of the preferred embodiment is a multi-layered tantalum assembly, sandwiched between layers of thermally conductive substrate. A coolant system draws heat generated by the target away from the substrate.
According to a more limited aspect of the invention, an optical sensor detects when product is present in the region and only allows the accelerator to release electrons when there is product in the region.
According to another aspect of the present invention, a product irradiation system is provided including an accelerator, a product conveyer, and an x-ray anode for the production of x-rays as a result of electrons generated from the accelerator striking it.
According to another aspect of the present invention, a method of x-ray production is provided where electrons encounter multiple layers of target material and are converted multiple spectra of x-rays.
According to another aspect of the present invention, an x-ray target is given made of layers of high Z material sandwiched between layers of thermally conductive low Z material which allow the propagation of heat away from the high Z material.
One advantage of the present invention is that it produces x-rays efficiently.
Another advantage of the present invention is that anode life is extended.
Another advantage of the present invention is that coolant corrosion of the target is eliminated.
Yet another advantage of the present invention resides in reduced heating.
Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.